ARACHNIDA. WJ 
Arachnida. 
No attempt will be made to pass in review all of the subclasses of the arachnids. Some 
of the Merostomata are so obviously trilobite-like that it would seem that their relationship 
could easily be proved. The task has not yet been satisfactorily accomplished, however, 
and new information seems only to add to the difficulties. 
So far as I know, the Araneas have not previously been compared directly with trilobites, 
although such treatment consists merely in calling attention to their crustacean affinities, as 
has often been done. 
Carpenter's excellent summary (1903, p. 347) of the relationship of the Arachnida to 
the trilobites may well be quoted at this point : 
The discussion in a former section of this essay on the relationship between the various orders of 
Arachnida led to the conclusion that the primitive arachnids were aquatic animals, breathing by means of 
appendicular gills. Naturally, therefore, we compare the arachnids with the Crustacea rather than with the 
Insecta. The immediate progenitors of the Arachnida appear to have possessed a head with four pairs of 
limbs, a thorax with three segments, and an abdomen with thirteen segments and a telson, only six of which 
can be clearly shown by comparative morphology to have carried appendicular gills. But embryological 
evidence enables us to postulate with confidence still more remote ancestors in which the head carried well 
developed compound eyes and five pairs of appendages, while it may be supposed that all the abdominal 
segments, except the anal, bore limbs. In these very ancient arthropods, all the limbs, except the feelers, 
had ambulatory and branchial branches ; and one important feature in the evolution of the Arachnida must 
have been the division of labour between the anterior and posterior limbs, the former becoming specialized 
for locomotion, the latter for breathing. Another was the loss of feelers and the degeneration of the com- 
pound eyes. Thus we are led to trace the Arachnida (including the Merostomata and Xiphosura) back to 
ancestors which can not be regarded as arachnids, but which were identical with the primitive trilobites,, and 
near the ancestral stock of the whole crustacean class. 
TRILOBITES NOT ARACHNIDA. 
While no one having any real knowledge of the Trilobita has adopted Lankester's scheme 
of the inclusion of the group as the primitive grade in the Arachnida, reference to it may 
not be amiss. This theory is best set forth in the Encyclopaedia Britannica, Eleventh 
Edition, under the article on Arachnida. It is there pointed out that the primitive arachnid, 
like the primitive crustacean, should be an animal without a fixed number of somites, and 
without definitely grouped tagmata. As Lankester words it, they should be anomomeristic 
and anomotagmatic. The trilobites are such animals, and he considers them Arachnida and 
not Crustacea for the following reasons : 
Firstly and chiefly, because they have only one pair (apart from the eyes) of pre-oral 
appendages. "This fact renders their association with the Crustacea impossible, if classifi- 
cation is to be the expression of genetic affinity inferred from structural coincidence." 
Secondly, the lateral eyes resemble no known eyes so closely as the lateral eyes of 
Limulus. 
Thirdly, the trilobation of the head and body, due to the expansion and flattening of 
the sides or pleura, is like that of Limulus, but "no crustacean exhibits this trilobite form." 
Fourthly, there is a tendency to form a pygidial or telsonic shield, "a fusion of the pos- 
terior somites of the body, which is precisely identical in character with the metasomatic 
carapace of Limulus." No crustacean shows metasomatic fusion of segments. 
Fifthly, a large post-anal spine is developed "in some trilobites" Che refers to a figure 
of Dalnianilcs). 
